Pump for transferring a liquefied gas

ABSTRACT

A pump for transferring liquefied gas, including a pump body and a wheel having blades rotatably mounted inside the pump body, the pump has magnets arranged at the periphery of the wheel, the pump further including a field winding arranged to drive the wheel in rotation with the magnets.

TECHNICAL FIELD

The present invention applies in particular to a device for transportingand/or storing a liquefied gas and to a method of transferring, ordelivering, the liquefied gas from the device.

The present invention applies in particular to containers fortransporting or storing refrigerated gas such as oxygen, nitrogen,argon, or liquefied natural gas, for example, which liquefied gas isstored at a pressure of about 10 bars to 20 bars approximately, or ofabout 3 bars to 30 bars approximately, for example.

STATE OF THE ART

Transferring a liquefied gas contained under pressure in a container toa gas storage tank, usually referred to as “unloading”, may be performedby making use of a pressure difference, with this mode of transportusually being referred to as “gravity unloading”, or else it may betransferred by pumping.

Gravity unloading requires the pressure in the container to be higherthan the pressure in the “client” tank for filling by an amount that issufficient to ensure that the liquefied gas travels along a transfercircuit connecting the container to the tank, solely under the effect ofthe difference between these two pressures.

During transfer, the pressure in the container drops as a result ofdischarging the liquid phase present in the bottom portion of thecontainer into the transfer circuit, and as a result the flow rate ofthe gas being transferred also drops.

In order to mitigate (at least in part) these drops in pressure and flowrate, it is known to provide the container with a recirculation circuitthat connects the bottom of the container to the top portion of thecontainer, which circuit includes a heat exchanger arranged to heat theliquid phase leaving the container via this circuit, so as to cause thisliquid phase to boil, this circuit returning with the gas phase thatresults from this boiling to the gas space inside the container, therebyincreasing the pressure inside the container.

The heat exchanger exchanges heat between the gas flowing through theheat exchanger and a heat source, which may be constituted by ambientair, for example, so that the heat exchanger may be referred to as anatmospheric heater.

The (partial) compensation for the drop in pressure inside the containeras provided by the recirculation (or heater) circuit also lessens duringtransfer as a result of the (progressive) lowering of the level of theliquid phase inside the container.

Consequently, unloading is usually performed by pumping using a pumpprovided in the transfer circuit that is used for feeding the tank withliquefied gas.

A pump for transferring liquefied gas, i.e. a cryogenic pump, generallycomprises a pump body and a bladed wheel mounted to rotate inside thepump body.

As described in patent FR 2 822 927, a fraction of the liquid phasedelivered by the pump may flow through a heat exchanger for evaporatingthis liquid phase fraction in order to compensate for the pressure dropinside the container, and also for condensing a gas phase taken from thegas space in a tank being filled from the container.

As described in patent FR 2 439 881, before being started, such a pumpmust be cooled down by using a “natural” (or “gravity”) flow of theliquefied gas through the pump, which generally requires an unwantedwaiting time that, under certain circumstances, may last for one or morehours before it is possible to start the pump.

Furthermore, in spite of using pump-starting methods and devices asdescribed in that patent FR 2 439 881, starting and operating the pumpcorrectly remain major sources of difficulty for an operator withordinary qualifications.

Specifically, transfer pumps operate correctly only in a narrow range ofpressures: below a minimum pressure the sealing members of the pumpleak; and above a maximum pressure that is not much higher than theminimum pressure, the sealing members and/or the pump suffer prematurewear.

Furthermore, transfer pumps are usually driven by an electric motor andconsume a large amount of energy.

SUMMARY OF THE INVENTION

An object of the invention is to propose a pump for circulatingliquefied gas, which pump improves and/or remedies, at least in part,the shortcomings or drawbacks of known pumps for transferring liquefiedgas.

An object of the invention is to propose a pump for transferringliquefied gas that requires little or no prior cooling before starting.

An object of the invention is to propose a device for transportingand/or storing liquefied gas and a method of transferring, ordelivering, liquefied gas from the device, that are improved and/or thatremedy, at least in part, the shortcomings or drawbacks of known systemsfor transporting, storing, and/or delivering liquefied gas.

In an aspect of the invention, there is provided a device fortransporting or storing liquefied gas under pressure, the devicecomprising:

-   -   a container for containing the liquefied gas under pressure;    -   a circuit for transferring the liquefied gas in the liquid        phase, which circuit is connected to the bottom portion of the        container, and includes a member for connection to a tank or to        a gas transport network that is to be fed with gas; and    -   a circuit for recirculating the liquefied gas, which circuit is        connected to the top portion of the container and includes a        heater and a recirculation pump that is connected in series with        the heater, upstream from the heater, and that is arranged to        deliver into the heater the pumped liquefied gas taken from the        bottom portion of the container so as to accelerate the        circulation of the liquefied gas through the heater, thereby        increasing heat exchange in the heater and maintaining, or        increasing, the pressure of the gas space in the container.

In another aspect of the invention, there is provided a method oftransferring liquefied gas under pressure contained in a container intoa tank or a gas transport network, wherein the container is connected toa recirculation circuit that includes a heater and arecirculation/heater pump that is connected in series with the heater,upstream from the heater, and that is arranged to discharge liquefiedgas taken from the bottom portion of the container into the heater, themethod comprising:

-   -   connecting the container to the tank or to the network that is        to be fed, via a circuit for transferring the liquefied gas in        the liquid phase and not having a pump;    -   allowing the liquefied gas to be transferred to the tank or to        the network via the transfer circuit under the effect of the        higher pressure in the container (in comparison with the        pressure that exists in the tank or the network), or ensuring        that it is so transferred; and    -   operating the recirculation pump to compensate, at least in        part, for the reduction in pressure inside the container during        transfer.

By way of example, the recirculation pump may be designed to recirculateliquefied gas at a rate lying in a range about 10 liters per hour (L/h)to about one thousand (1000) L/h, or a range about 20 L/h to about 2000L/h, and to provide a pressure rise (pressure head) lying in a rangeabout one-tenth of a bar (0.1 bar) to about one bar.

In order to maintain sufficient pressure inside the container whiletransferring the liquefied gas, in particular in order to maintain apressure inside the container that is substantially constant throughoutthe transfer, it is possible to measure the pressure that exists insidethe container and to control the operation of the pump as a function ofthe measured pressure.

For this purpose, the transport or storage device may include apressure-measurement sensor arranged to measure the pressure that existsinside the container, and a control unit connected to the pump and tothe pressure-measurement sensor and arranged/configured, in particularprogrammed, to control the operation of the pump as a function of thepressure measured by the sensor.

The invention applies in particular to storage containers that arethermally insulated and that are supported by a transport structureforming part of the gas transport and storage device, in particular bymeans of a road transport trailer or by a structure for transport byroad, by rail, or by boat, where such a structure generally includesframes in the ISO standard format.

The container may be elongate in shape along a horizontal axis.

The recirculation circuit may include a check valve arranged downstreamfrom the heat exchanger and serving to prevent the gas phase escapingfrom the container into the circuit when the pump is stopped.

In accordance with another aspect of the invention, in order to causethe liquid phase to circulate in the recirculation circuit, it ispossible to make use of a axial flow pump in which the bladed wheel haspermanent magnets arranged at the periphery of the wheel, the pumpfurther including a field winding arranged to drive the wheel inrotation by means of the magnets.

The gas transport or storage device may include electrical energystorage means such as a battery for powering the pump, and in particularthe field winding of the pump.

Under such circumstances in particular, the gas transport or storagedevice may also include energy capture means, such as photovoltaiccells, serving to power the energy storage means and/or the pump.

The electrical energy capture and storage means may be secured to, andsupported by, the transport structure and/or the container.

The field winding of the pump may be powered by an electrical powersupply and control unit that is connected to the field winding.

This unit and the field winding may be arranged to drive the pump wheelat a speed of rotation lying in the range about 1000 revolutions perminute (rpm) to about 5000 rpm, and in particular in a range about 1500rpm or 2000 rpm to about 4000 rpm or 4500 rpm.

Driving the wheel by the magnetic effect serves in particular to limit,or to avoid, any rise in the temperature of the wheel while the pump isstopped, and consequently facilitates subsequent starting.

The field winding is preferably arranged outside the pump body. Inparticular, the field winding, and/or the coil of the field winding, mayextend facing the magnets and the periphery of the wheel.

The coil of the field winding may be embedded in an electricallyinsulating material such as a polymer material, such that the coil andthe field winding can withstand ice forming on the pump.

The insulating material may be a thermally insulating material, suchthat the pump body is heated little by the field winding.

The invention also makes it possible to provide and use a pump that iscompact and that provides good performance in circulating liquefied gas,that requires little or no cooling prior to operation, and that iseasier for a relatively unqualified operator to control.

At least a portion of the bladed wheel, in particular a peripheralstructure or portion connecting together the tips of the blades,specifically a peripheral structure of substantially annular shape, maybe made out of a magnetic material, in particular out of (ferro)magneticstainless steel (martensitic or ferritic steel, in particular), in orderto facilitate driving the wheel by means of a magnetic field produced bythe field winding.

Also for this purpose, the magnets may be secured to such a peripheralportion of the wheel and they may be arranged so as to be substantiallyflush with the peripheral envelope or envelope surface of the wheel.

The magnets may in particular present the shape of a portion of acylindrical cap, with a radius of curvature that matches, in particularis substantially equal to, the outside radius of the wheel, so as tominimize the airgap between the magnets and the field winding.

The magnets may be secured to the wheel by friction and/or abutment, inparticular by mechanical blocking or by crimping, so as to avoid usingan adhesive that might react with the liquefied gas passing through thepump.

The bladed wheel may be mounted to rotate freely on a stationary shaft,or pivot, that is rigidly connected to the pump body by a connectionstructure that is provided with or pierced by openings allowing theliquefied gas to pass through the structure.

The connection structure may include, or may be essentially constitutedby a grid of stationary vanes that may be arranged downstream from thewheel and that may serve to guide the fluid flow so as to improve theefficiency of the pump.

This connection structure may present thermal conductivity that is lessthan the thermal conductivity of the pump body so as to limit the extentto which the wheel is heated by conduction during periods when the pumpis stopped, thereby reducing any need for the pump to be cooled downbefore starting.

For this purpose, at least a portion of the connection structure may benon-metallic, and in particular it may be made of a synthetic orplastics material such as polytetrafluoroethylene (PTFE).

The connection structure may in particular present thermal conductivitythat is less than the thermal conductivity of the wheel, and/or than thethermal conductivity of the shaft.

The pump, and in particular the connection structure, may include astatic sealing member suitable for providing the pump body with sealing.

The sealing member may include, or may be essentially constituted by, athin structure of annular shape such as a thin ring forming a flatgasket, which structure is arranged to provide sealing between twoportions of the pump body, each of which has a respective flange,sealing being provided when the two flanges are placed facing each otherwith the thin sealing structure clamped between them.

The connection between the wheel and the stationary shaft may take placevia a single bearing, e.g. a needle bearing or a composite metal/polymerbearing such as a bearing of PTFE coated bronze, thereby contributing tomaking the pump compact.

The pump body may include a central tubular structure defining a chamberin which the wheel is received. At least a portion of the tubularstructure, which extends between the field winding and the periphery ofthe wheel, may be made out of a non-magnetic material, in particular outof non-magnetic stainless steel.

The chamber may be in the form of a cylinder of diameter that matchesthe diameter of the wheel (i.e. is a little greater than the diameter ofthe wheel).

The pump body may also include two flared portions, e.g. ofsubstantially frustoconical shape, that are arranged at opposite ends ofthe central tubular structure and in line with the tubular structure,i.e. substantially on the same axis, such that the assembly defines afluid flow passage presenting few edges or discontinuities and/orpresenting a flow section that varies substantially continuously, thustending to prevent bubbles of gas forming in the pump body.

In another aspect of the invention, there is provided a method ofpumping a liquefied gas having a temperature situated in a range fromabout minus two hundred degrees Celsius (−200° C.) to about minus fiftydegrees Celsius (−50° C.) wherein use is made of a pump as claimed anddescribed in this application.

Other aspects, characteristics, and advantages of the invention appearfrom the following description, which refers to the accompanying figuresand shows preferred embodiments of the invention without any limitingcharacter.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram of a device for transporting and storing liquefiedgas.

FIG. 2 is an exploded diagrammatic perspective view showing a pump fortransferring/circulating liquid gas.

FIG. 3 is an exploded diagrammatic perspective view in perspective fromanother viewpoint showing the pump of FIG. 2.

FIG. 4 is a diagrammatic longitudinal section view on a larger scaleshowing the central portion of a pump similar to that of FIGS. 2 and 3.

DETAILED DESCRIPTION OF THE INVENTION

Unless stated explicitly or implicitly to the contrary, elements ormembers that are structurally or functionally identical or similar areidentified in the various figures by identical references.

Unless stated explicitly or implicitly to the contrary, the terms“upstream” and “downstream” are used relative to the flow direction ofthe liquefied gas.

With reference to FIG. 1, the device 10 serves for transporting, andwhere applicable for storing, a liquefied gas 29, 30 under pressure.

For this purpose, the device 10 has a container 12 of elongate shapewith an axis 13 that is substantially horizontal, which container isthermally insulated.

In order to transport gas, the container 12 is movable so as to besuitable for taking to the proximity of a tank 25 that is to be filledwith liquefied gas.

In the embodiment shown in FIG. 1, the tank 25 is of elongate shape withan axis 26 that is substantially vertical, and it is connected to thedevice 10 by a connection member 24 that is essentially constituted by aliquefied gas transport duct 24, which may be a flexible hose.

For (temporary) storage of gas, the container 12, which may bestationary, may be connected to a gas transport network 24 that is to besupplied with gas.

In the container 12, the gas phase 30 overlies the liquid phase 29 ofthe liquefied gas that can be maintained in the container at atemperature situated in a range from about minus two hundred degreesCelsius (−200° C.) to about −50° C., for example.

The container 12 is mounted on a road transport trailer suitable formoving the container 12, the trailer being represented diagrammaticallyin FIG. 1 by wheels 14.

Alternatively, the container 12 may be secured to a structure (notshown) for transport by boat, which structure may be incorporated in thevolume of an ISO container, for example.

In order to feed the tank 25 or a gas transport network with liquefiedgas, the device 10 has a transfer circuit for transferring liquefied gasin the liquid phase.

This circuit comprises a duct 17 that is connected to the bottom to thecontainer 12 and opens out in the container at a first end of the duct17.

The transfer circuit also has an isolation valve 18 located at a secondend of the duct 17 and enabling the duct to be closed.

The duct 24 for connecting the device 10 to the tank 25 extends the duct17 beyond the valve 18.

The device 10 also has a liquefied gas recirculation circuit that isconnected to the top portion of the container and that opens out intothe container via an end of a duct 22 forming part of this circuit.

The recirculation circuit comprises the following items successivelyconnected together in pairs (i.e. in series):

i) a duct 19 connected to the duct 17;

alternatively, the duct 19 may be connected to the bottom portion of thecontainer 12 by opening out into the container;

ii) a recirculation pump 15 connected to the duct 19 via its suctionorifice and designed to suck in liquefied gas in the liquid phasetransported by this duct;

iii) a duct 20 connected to the delivery orifice of the pump 15;

iv) a heater 11 connected to the duct 20 to receive the liquefied gasdelivered by the pump and transported by the duct 20 in order toevaporate the gas by exchanging heat with a heat source such as ambientair; and

v) a duct 22 for transporting the gas exhausted from the heater 11,generally in gaseous form, and taking it to the container 12, which ductmay be fitted with a check valve 21 preventing the gas phase 30contained in the container 12 from being exhausted towards the heater11.

As described in detail below with reference to FIGS. 2 to 4, the axialflow pump 15 comprises a pump body, a bladed wheel mounted to rotateinside the pump body, and an electric motor for driving the wheel inrotation.

The motor comprises a permanent magnet armature secured to the wheel anda field winding arranged outside the pump body.

The bladed wheel is of the “helical” or “axial” type serving to move thepumped liquefied gas substantially along the axis of rotation of thewheel, which axis coincides substantially with an axial axis of symmetryof the pump body.

The bladed wheel has a row of blades arranged in annular cascade.

The motor of the pump may be powered electrically by an electricitydistribution network to which the motor and the pump 15 is connected.

Alternatively, or in addition, the motor may be powered electrically bya battery 32 for storing electrical energy that may be secured to thetransport structure and/or to the container 12, and that is connected tothe motor of the pump 15.

In this configuration in particular, the device 10 may includephotovoltaic solar cells 31 serving to feed electricity to the storagebattery 32 and/or to the pump 15, and that may be secured to thetransport structure and/or to the container 12.

In order to enable the operation of the pump to be controlled so as toensure a determined pressure inside the container 12 while the tank 25is being filled, the device 10 may include a pressure-measurement sensor23 arranged to measure the pressure that exists in the gas phase insidethe container 12, and a control unit 16 connected to the pump 15 and tothe sensor 23 and arranged, in particular programmed, to control theoperation of the pump 15 as a function of the pressure measured insidethe container 12.

In this respect, when the recirculation pump is stopped (deenergized),recirculation of gas may take place in the recirculation circuit, at alow flow rate, due to gas boiling in the heater 11, assuming the circuitvalves are open.

Energizing the pump to rotate the bladed wheel in a first direction ofrotation will result in a higher recirculation flow rate, whileenergizing the pump to rotate in a second (opposite) direction ofrotation will result in a lower or zero recirculation flow rate.

With reference to FIGS. 2 to 4, the cryogenic pump 15 comprises a pumpbody 70, 80, 100 and a wheel 41 having blades 45 that is mounted torotate inside the pump body, about an axis of rotation 40, which is ageneral axis of symmetry of the pump and most of the parts making it up.

The pump body has a central tubular structure 100 defining a cylindricalchamber 110 that receives the wheel 41, the chamber 110 beingcylindrical in shape about the axis 40 and of inside diameter that isslightly greater than the outside diameter of the wheel.

The wheel 41 has magnets 42 arranged at the periphery of the wheel andregularly spaced apart.

The wheel 41 has an annular peripheral ring 44 connected to the tip ofthe blades 45 that are surrounded by the ring.

The magnets 42 are secured to the peripheral ring 44 of the wheel andthey are arranged to be substantially flush with the peripheral envelopeof the wheel.

Each magnet 42 is in the form of a portion of a cylindrical cap with aradius of curvature that matches the outside radius of the ring 44and/or of the wheel 41.

On its outside face, the ring 44 has notches 43 of identical shape anddimensions matching the magnets, which are regularly spaced apart aroundthe outline of the wheel.

The magnets 42 are inserted in the notches 43 and they are held in placeby means of a second ring 46 of diameter matching that of the first ring44 so that the ring 46 can be secured to the first ring 44, e.g. bycrimping, so as to ensure that the magnets are mechanically fastened tothe periphery of the wheel.

At least a portion of the wheel 41, and in particular the rings 44 and46, is/are preferably made of a magnetic material such as aferromagnetic stainless steel.

The wheel 41 is mounted to rotate freely about the axis 40 inside thechamber 110 on a stationary shaft 50.

The shaft 50 has a cylindrical bearing surface 51 of axis 40 having arolling bearing 90 engaged thereon, e.g. a needle bearing, whichconstitutes a bearing for the wheel 41, allowing the wheel to rotatefreely on the shaft 50.

Each end 52, 54 of the shaft 50, which extends from upstream todownstream relative to the wheel 41, i.e. from left to right relative tothe wheel in FIGS. 2 and 3, presents a streamlined shape for guiding theliquefied gas both before and after its passage through the wheel.

The shaft 50 is rigidly connected to the pump body by a connectionstructure 60 pierced by openings 61 that enable the liquefied gas topass through the structure.

The connection structure 60 is fastened to the second end 53 of theshaft 50 and extends downstream from the wheel 41.

The structure 60 comprises a grid of stationary vanes 62 defining theopenings 61, possibly serving to guide the flow, and connecting acentral portion of the structure 60 to an annular peripheral portion ofthe structure 60.

The pump also has a field winding 120 serving to produce a magneticfield for driving the wheel in rotation by means of the magnets.

The tubular structure 100, which extends between the field winding andthe periphery of the wheel, is made of a non-magnetic material.

The field winding 120 is arranged outside the pump body, facing andaround the tubular structure 100, and facing and close to the magnetssituated at the periphery of the wheel.

The coil of the field winding is embedded in an insulating material 121,thus keeping the coil separate from the tubular wall 100 forming aportion of the pump body.

The pump body also has two tubular segments 70 and 80 having tworespective flared portions 72 and 82 that are arranged on either side ofand in line with the central tubular structure 100.

The connection structure 60 has a collar 63 lying in a planeperpendicular to the axis 40 and projecting outwards from the chamber110 that receives the shaft 50, the wheel 41, and a substantial fractionof the structure 60.

The collar 63 forms a static sealing member for providing sealingbetween the two portions 70, 80, and 100 of the pump body, which arethemselves provided with respective flanges 71 and 81, sealing beingprovided when these two flanges are arranged facing each other and areassembled together by bolts 91, 92, clamping the collar 63 between them,as shown in FIG. 4.

1. A pump for transferring liquefied gas, comprising: a pump body; awheel having blades rotatably mounted inside the pump body, each bladehaving a tip, the bladed wheel having permanent magnets arranged at theperiphery of the wheel; a field winding comprising a coil and arrangedto drive the wheel in rotation with the magnets, the coil being embeddedin an electrically insulating material; and a sealing structure ofannular shape arranged to provide sealing between two portions of thepump body, each of which portions is provided with a respective flange,sealing being provided when the two flanges are placed facing each otherwith the sealing structure clamped between them.
 2. The pump accordingto claim 1, wherein the field winding is arranged outside the pump body,facing the magnets and the periphery of the wheel.
 3. The pump accordingto claim 1, wherein the bladed wheel comprises a peripheral structureconnecting together the tips of the blades, the magnets being secured tothe peripheral structure and arranged so as to be substantially flushwith a peripheral envelope of the wheel.
 4. The pump according to claim3, wherein the peripheral structure is made out of a magnetic material,in particular out of a (ferro)magnetic stainless steel.
 5. The pumpaccording to claim 1, wherein the magnets are in the form of portions ofa cylindrical cap with a radius of curvature that matches the outsideradius of the wheel.
 6. The pump according to claim 1, wherein themagnets are secured to the wheel by friction and/or abutment, inparticular by mechanical blocking, or by crimping.
 7. The pump accordingto claim 1, wherein the bladed wheel is mounted to rotate freely on astationary shaft rigidly connected to the pump body by a connectionstructure that is pierced by openings enabling liquefied gas to passthrough the connection structure.
 8. The pump according to claim 7,wherein the connection structure includes a grid of stationary vanesarranged downstream from the wheel and serving to guide the gas flow. 9.The pump according to claim 7, wherein the connection structure presentsthermal conductivity that is less than the thermal conductivity of thewheel, than the thermal conductivity of the pump body, and/or than thethermal conductivity of the shaft.
 10. The pump according to claim 7,wherein the connection structure includes the sealing structure ofannular shape.
 11. The pump according to claim 7, wherein the connectionbetween the wheel and the stationary shaft is provided by a singlebearing.
 12. The pump according to claim 1, wherein the wheel serves tomove the pumped liquefied gas substantially along an axis of rotation ofthe wheel.
 13. The pump according to claim 1, wherein the pump bodyincludes a central tubular structure defining a chamber in which thewheel is received, at least a portion of the tubular structure, whichextends between the field winding and the periphery of the wheel, beingmade out of a non-magnetic material, the chamber presenting acylindrical shape of diameter that is little greater than the diameterof the wheel.
 14. The pump according to claim 13, wherein the pump bodyalso has two flared portions arranged on either side of and in line withthe central tubular structure.
 15. A cryogenic axial flow pumpcomprising: a pump body; a wheel having a row of blades arranged inannular cascade, each blade having a tip, the bladed wheel beingrotatably mounted inside the pump body and having permanent magnetsarranged at the periphery of the wheel, the bladed wheel being mountedto rotate freely on a stationary shaft rigidly connected to the pumpbody by a connection structure that is pierced by openings enablingliquefied gas to pass through the connection structure, the bladed wheelserving to move pumped liquefied gas substantially along an axis ofrotation of the wheel; a field winding comprising a coil and arranged todrive the wheel in rotation with the magnets, the coil being embedded inan electrically insulating material; and a sealing structure of annularshape arranged to provide sealing between two portions of the pump body,each of which portions is provided with a respective flange, sealingbeing provided when the two flanges are placed facing each other withthe sealing structure clamped between them.
 16. The pump according toclaim 15, wherein the field winding is arranged outside the pump body,facing the magnets and the periphery of the wheel.
 17. The pumpaccording to claim 15, wherein the bladed wheel comprises a peripheralstructure connecting together the tips of the blades, the magnets beingsecured to the peripheral structure and arranged so as to besubstantially flush with a peripheral envelope of the wheel.
 18. Thepump according to claim 17, wherein the peripheral structure is made outof a magnetic material such as a (ferro)magnetic stainless steel. 19.The pump according to claim 15, wherein the magnets are in the form ofportions of a cylindrical cap with a radius of curvature that matchesthe outside radius of the wheel.
 20. The pump according to claim 15,wherein the magnets are secured to the wheel by friction, by abutment,by mechanical blocking, or by crimping.
 21. The pump according to claim15, wherein the connection structure includes a grid of stationary vanesarranged downstream from the wheel and serving to guide the gas flow.22. The pump according to claim 15, wherein the connection structurepresents thermal conductivity that is less than the thermal conductivityof the wheel, than the thermal conductivity of the pump body, and/orthan the thermal conductivity of the shaft.
 23. The pump according toclaim 15, wherein the connection structure includes the sealingstructure of annular shape.
 24. The pump according to claim 15, whereinthe connection between the wheel and the stationary shaft is provided bya single bearing.
 25. The pump according to claim 15, wherein the pumpbody includes a central tubular structure defining a chamber in whichthe wheel is received, at least a portion of the tubular structure,which extends between the field winding and the periphery of the wheel,being made out of a non-magnetic material, the chamber presenting acylindrical shape of diameter that is little greater than the diameterof the wheel.
 26. The pump according to claim 25, wherein the pump bodyalso has two flared portions arranged on either side of and in line withthe central tubular structure.
 27. A method of pumping a liquefied gashaving a temperature situated in a range from about minus two hundreddegrees Celsius (200° C.) to about minus fifty degrees Celsius (50° C.)wherein use is made of a pump according to claim 1.